Information AboutPopulation I Stars |
| CATEGORIES ABOUT METALLICITY | |
| astrophysics | |
| physical cosmology | |
| stellar astronomy | |
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In Astronomy and Physical Cosmology , the metallicity of an object is the proportion of its matter made up of Chemical Element s other than Hydrogen and Helium . (This terminology is used differently to the usual meaning of the word 'metal' - since on the grandest of scales the Universe is overwhelmingly composed of hydrogen and helium, astronomers label all the Heavier elements "metal").1 For example, a Nebula rich in Carbon Compounds would be called "metal-rich", even though Carbon is not considered a metal in other contexts. The metallicity of an astronomical object may provide an indication of its age. When the universe first formed, according to the Big Bang theory, it consisted almost entirely of hydrogen which, through Primordial Nucleosynthesis , created a sizeable proportion of helium and only trace amounts of Lithium . The first Stars , referred to as Population III, had virtually no metals at all. These stars were incredibly massive and, during their Lives , created the elements up to Iron in the Periodic Table via Nucleosynthesis . They subsequently died in spectacular Supernovae which dispersed those elements throughout the universe. As of 2007 , no Population III stars have been found; rather, their existence is inferred in current Models of the Origin Of The Universe . The next generation of stars was born out of those materials left by the death of the first. The oldest observed stars, known as '''Population II''', have very low metallicities;2 as subsequent generations of stars were born they became more metal-enriched, as the Gas eous clouds from which they formed received the metal-rich Dust manufactured by previous generations. As those stars died, they returned metal-enriched material to the Interstellar Medium via Planetary Nebula e and supernovae, enriching the nebulae out of which the newer stars formed ever further. These youngest stars, including the Sun , therefore have the highest metal content, and are known as '''Population I''' stars. Across the , two small Irregular Galaxies Orbit ing the Milky Way, the Large Magellanic Cloud has a metallicity of about forty per cent of the Milky Way, while the Small Magellanic Cloud has a metallicity of about ten per cent of the Milky Way. CALCULATION The metallicity of the Sun is approximately 1.6 percent by mass. For other stars, the metallicity is often expressed as " {Link without Title} ", which represents the Logarithm of the ratio of a star's iron abundance compared to that of the Sun. The formula for the logarithm is expressed thus: Here and is the number of iron and hydrogen atoms per unit of volume respectively. By this formulation therefore, stars with a higher metallicity than the Sun have a positive logarithmic value, while those with a lower metallicity than the Sun have a negative value. The logarithm is based on Powers Of Ten ; stars with a value of +1 have ten times the metallicity of the Sun (101), while those with +2 have a hundred (10&2) and those with +3 have a thousand (10³). Conversely, those with a value of -1 have one tenth (10 -1), while those with -2 have a hundredth (10-2) and so on. John C. Martin: What we learn from a star's metal content. Young Population I stars have significantly higher iron-to-hydrogen ratios than older Population II stars. Primordial Population III stars are estimated to have a metallicity of less than −6.0, that is, less than a millionth of the abundance of iron which is found in the Sun. POPULATION I STARS Population I or metal-rich stars are those young stars whose metallicity is highest. The Earth 's Sun is an example of a metal-rich star. These are common in the Spiral Arm s of the Milky Way galaxy. |
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